On the investigation of energy saving aspects of commercial lifts

This paper investigates energy efficiency issues in modern lifts, based on the VDI 4707 guidelines, in the context of “KLEEMANN-LESS” research project, funded by national resources and the EU. The above analysis is applied to various lift types manufactured by the Greek multinational company ΚLEEMAN HELLAS. The obtained results indicate the relationship between critical technical parameters, such as the elevator driving system type and the standby energy consumption, according to the aforementioned standard. Furthermore, in the current work, new techniques for energy savings are proposed, employing realistic scenarios, which significantly enhance energy efficiency. Experimental work shows that an energy saving of up to 40% can be achieved. The outputs of the current work are not limited to lift models manufactured by KLEEMAN HELLAS, but concern the majority of lift manufacturers as well.

energy classes for lifts have been introduced, especially in countries which are significant producers of these systems (Germany, France, etc). Consequently, the European market is expected to become inaccessible for elevator industries that have not anticipated the installation of appropriate systems for restraining energy consumption.
The adjustment of energy consumption and classification of modern lifts is performed according to VDI 4707 (2007). The energy demand of lifts can be distinguished on the basis of standby demand and travel demand. Standby demand is the total energy demand of the lift in standby mode, meaning that only parts of the electrical equipment and components that contribute to the lift's readiness for operation need to be considered. The travel demand is the total energy demand of the lift during travels at specific trip cycles and with specific loads (VDI 4707 2007).
In Barney (2003), CIBSE (2005), Howkins (2006), ISO 18738 (2003, Nipkow (2005), and Spyropoulos and Asvestopoulos (2008), the energy waste is defined as the ratio of the standby demand of the lift to the total annual energy consumption. However, the reduction of the energy consumption and the corresponding certification mandate the adoption of sophisticated algorithms for the control and supervision of the system; in order to ensure the efficiency of the above methodologies, the technical characteristics and real usage of the lifts must be taken into account. In addition, energy saving can be achieved using regenerative braking systems. These techniques are already applied in high speed lifts (for multilevel buildings), but their efficient application to different types of lifts is dependent on the appropriate design, in a way that an adequate level of energy recovery is obtained and, simultaneously, improvement of the lift energy class is performed.
The aim of the current work is the presentation of methods for the design and the application of the appropriate energy saving techniques, as far as the operation of various types of modern lifts is concerned. The experimental measurements performed indicate that an upgrade of the energy class is a realistic target, since the energy saving can reach up to 40%.
Note that the current analysis was performed using commercial lifts manufactured by KLEEMAN HEL-LAS (which is a significant manufacturer of the global lifts' market), but it can be also applied to other manufacturers. So, our work could be the beginning for the development of a policy that focuses on the reduction of the energy consumption for this vertical transportation vehicle.

Energy efficiency of lifts
The VDI 4707 guideline distinguishes the energy consumption of the lifts on the basis of standby energy demand E SB and travel energy demand E L (VDI 4707 2007), according to the following equations: where S nom (m), the annual travel distance. Q (kg), nominal load. k, a load factor in order to take into account the lift loading during the calculation of E travel , spec . P standby (W), standby demand. The specific travel energy demand is given as: where E travel , spec (mWh/kg . m), specific travel energy demand.
E refer , trip (Wh), measured travel energy demand for a reference trip.
L (m), the reference trip length (it depends on the elevator usage type).
Finally, the specific energy demand E elev , spec , max of the lift is given as: The lift is assigned to energy demand classes according to the demand values for standby and travel. According to Eq. (6), the energy efficiency classes for a lift are determined from the energy demand values for standby and travel by projecting with the average standby times and travel times for a daily demand and then dividing it by the number of meters traveled and the nominal load. An additional parameter in this classification is the usage category of the elevator, indicating its average daily travel. The exact classification process can be found in the VDI 4707 guideline document.
Moreover, the recently published International Standard ISO 25745-2:2015, 2015 is an alternative reliable method to estimate and compare the energy performance of elevators. Nevertheless, the widely used VDI 4707 is still the prominent method for the calculation of the energy consumption. So, for the scope of the specific work (where we are concentrating on the energy behavior of lifts as well as on the search for possible technical solution that may improve their energy efficiency), it is rather convenient to use the existing data, which cover the whole range of KLEEMANN HELLAS commercial lifts.

Energy study for various lift types of KLEEMAN HELLAS
The current section presents the energy consumption characteristics of various types of lifts, produced by KLEEMAN HELLAS.  (VDI 4707 2007). It is worth noting that traction elevators outperform the hydraulic ones, as expected. However, some hydraulic elevators that utilize an inverter drive, i.e., Nos. 7, 12, and 13, are highly classified, thanks to the electronic regulation of their energy consumption (E4 Project 2010; Yang et al. 2007).

Energy consumption results
The current section presents the most important indices-targets for the reduction of the energy consumption for all lift types of KLEEMAN HELLAS. The upgrade of the energy efficiency class (ideally, class A would be the optimum situation) is the basic criterion of the performed analysis. Figure 1 presents the existing energy efficiency class of the various lift types according to VDI 4707. The majority of the hydraulic lifts are categorized as class C and the majority of the traction lifts are included in class B. This is due to the fact that the traction lifts (except for No. 14) use driving inverters, which limit the energy demand of the lift during travels. Furthermore, the travel demand of high speed and vertical distances lifts (No. 2,No. 3) is even less, due to the use of an additional regenerative braking inverter. However, their upgrade to class A is not achieved, because the standby demand is high (column g in Table 1). Figure 1 highlights the fact that there are upgrading opportunities for all the lifts in terms of energy efficiency class (apart from lifts No. 13 and No. 14, which are in class A and have limited applicability due to their old technology). It is worth mentioning that the energy efficiency upgrade is a significant competitive advantage, even for lifts with high energy consumptions.
Next, we examine the potential for each lift type for upgrade by limiting its energy consumption. Figure 2 presents the ratio of the annual standby demand to the annual travel demand, E SB /E L , considering frequent (1 h per day), medium (0.5 h per day), and seldom (0.2 h per day) usage; these average hours of operation per day are consistent with the VDI 4707 limits. Obviously, in any case, t standby are the cumulative hours a lift is off duty, on annual basis. E L is calculated by using Eq. (1), while k, Q, and E travel,spec values are provided by Table 1. Similarly, E SB is calculated by using Eq. (2); P standby values are also provided in Table 1. According to Fig. 2, E SB /E L increases extremely for lifts with seldom or medium usage intensity, independent of their technology.
This happens due to the fact that standby energy consumption of the electronic circuits, remains high for many hours per day, even if it does not exceed 120 W. The results of lifts Nos. 12 and No. 15, where the standby energy consumption is 20 W, the consumption ratio is 50 and 250%, respectively, indicate that additional techniques must be applied for the reduction of the standby demand.  Fig. 1 The energy class of the lifts under study Figure 3 presents the necessary percentage reduction of the standby and travel energy consumption, ΔE, in order to achieve the energy efficiency class upgrade to the next upper class. E tot is the sum of E SB and E L , while the category usage (and so the definition of t L ) for each elevator type is set by the manufacturer and is the same that has been used for their categorization according to VDI 4707. Note that t L is assumed to be the upper boundary value of the respective usage category (according to VDI 4707). Additionally, Fig.4 depicts the necessary percentage reduction of the standby and travel energy consumption (under the same conditions with the ones in Fig. 3), in order to achieve the energy efficiency class upgrade of the lifts to class A.
The performed analysis indicates that a small reduction of the energy consumption can lead to an energy efficiency class upgrade (even if the reduction is 5%). However, the achievement of class A demands a significant restraint of the energy consumption (25-30%). In any case, the research for technical solutions to this effect is meaningful, considering the results in Fig. 2. (1) Hydraulic liŌs (2)  Indeed, although the needed percentage reduction of energy consumption (see Figs. 3 and 4) is, in many cases, higher than 30%, it seems that the improvement of the energy behavior of the lifts relates to the limitation of standby consumptions, especially for the cases of seldom and medium usage intensity. Figure 5 presents the expected energy efficiency class upgrade (to the next class) for each lift type, considering only the limitation of standby consumption, ΔE SB . The reference E SB value is set for each elevator type under the same conditions with the ones in Figs 3 and 4; according to these calculations, it seems that in some cases, the needed reduction of standby consumption is approximately 10%, a percentage that is regarded as possible. However, if a higher reduction of the standby consumption is sought (30-40%), in combination with a smaller reduction of the travel demand, then the energy efficiency upgrade of all lifts under examination will have been achieved.

(1) (2) (3) (4) (5)
Upgrade of the lifts to energy class A Reduction of the power absorbed by electronic/ roundabout circuits The reduction of the power absorbed by electronic/ roundabout circuits can be performed as following: A.1) Redesign of the power supply circuits, in order to achieve a higher efficiency; all elevator electronic circuits are supplied through linear power supplies. Despite the fact that these supplies are very reliable, they operate with low efficiency. For this reason, the linear power supplies must be replaced by high efficiency switching power supplies, which nowadays present very high reliability as well. This way, the reduction of the energy consumption during standby mode can reach approximately 20%. A.2) Replacement of old integrated circuits with new ones, which present lower energy consumption; the proposed solution can achieve significant energy saving, but may give rise to several compatibility issues; in case that the new integrated circuits have not a compatible pinout, then a new design of the electronic cards is needed, an option which cannot be recommended due to the investments already made by the manufacturing company in legacy circuitry.
Reduction of the energy consumption using intelligent techniques that break the standby operation B) Technical solutions for the reduction of the travel energy consumption KLEEMAN HELLAS has already installed driving systems with electronic voltage inverters in traction and hydraulic lifts of high net weight. In addition, the installation of inverters that utilize the braking energy in high speed lifts results to a reduction of the travel demand from 40 to 50% (Ceuca et al. 2010;Emadi et al. 2008;Minav et al. 2011;Mutoh et al. 2007;Oliveira et al. 2011;Papanikolaou et al. 2013;Yang et al. 2007;Yang et al. 2009;). However, the improvement of the travel energy consumption is also necessary; to this direction, the following technical solutions are proposed: B.1) Increase of the efficiency of the electrical motor drives: the efficiency of the existing electrical motors varies from 70 to 80%. Thus, the limitation of the travel demand can be achieved by using motors with higher efficiency. In addition, the selection of machines with lower nominal rpm and consequently lower gear conversion ratio will limit the travel demand. Therefore, the  installation of three phase induction as well as permanent magnet synchronous motors with high number of poles (more than 8) is recommended. The above changes can reduce the consumption up to 5%, according to calculations based on similar commercial lifts (Shah et al. 2012;Lu et al. 2011;Inoue et al. 2008). B.2) Development of power electronic converters for the utilization of the braking energy: this proposal concerns low net load hydraulic and traction lifts, which do not use systems for the utilization of the braking energy. This is due to the fact that the inverters in use deliver the energy to the electrical network, thus the energy losses are high. In contrast, the storage of braking energy, using battery banks or supercapacitors and its subsequent use by the lift is proposed (Azib et al. 2011;Marchesoni and Vacca, 2007;Mitronikas et al. 2014;Zhang et al. 2011). This solution limits the energy losses-the travel demand is reduced at least 10% for lifts of low net weight, according to Mitronikas et al. (2014) and Papanikolaou et al. (2013), and the system becomes financially sustainable. The energy efficiency figures for traction elevator types concerning B2 technical solution have been calculated through simulations of the supercapacitor energy storing solution presented in Mitronikas et al. (2014), taking into account the operational and technical characteristics of the elevator type. Additionally, indicative experimental tests have been carried out in KLEEMANN HELLAS installations, in order to confirm the simulation models. For this purpose, a prototype braking energy system with a 2.06 F/400 V supercapacitor bank, as intermediate energy storage unit, has been developed, that is charging/discharging through a high frequency (20 kHz) bidirectional converter, capable of handling the braking energy of traction lifts with nominal power up to 7.5 kW; the schematic diagram of this system is presented in Mitronikas et al. (2014). It is noted that these experimental  (Yang et al. 2007), taking into account the operational and technical characteristics of any single hydraulic elevator type. B.3) Reduction of the travel energy consumption by applying intelligent techniques for the limitation of the daily travel distance: this proposal concerns lifts of high usage intensity. An appropriate decision-making system defines the service priority, based on the minimization of the daily travel distance of the lift, taking into consideration real recorded data (Zarikas et al. 2013). As for A3 technical solution, the energy efficiency calculations have been performed through simulations of this Bayesian Network solution (taking into account the operational and technical characteristics of the elevator type). Figures 6,7,8,9,10,11,12,13,14, and 15 present the evaluation of the energy saving for each type of lifts manufactured by KLEEMAN HELLAS (except lifts No.13-15 due to their old technology). In more details, for any single commercial elevator, two energy saving scenarios are illustrated, i.e., a minimum and a maximum energy saving scenario. The estimation of the energy saving figures in both scenarios is based on the abovementioned technical solutions and their assessment in terms of energy saving, according to the processes that are described above; it is noted that this is not a straightforward procedure, because (among other parameters) the finalization of these figures calls for many technical details for each elevator subsystem. For instance, the estimation of the energy saving that may be achieved due to the redesign of the power supply circuits (A1 technical solution) is based on power consumption calculations according to the available data (commercial data sheets) of the electronic components that are actually used.

Conclusions
In the current study, the energy consumptions of various types of modern lifts are analyzed in-depth, highlighting quantitative and qualitative aspects of the obtained measurements. The analysis is performed for different lift types manufactured by KLEEMAN HELLAS. The energy consumption was distinguished on the basis of the standby travel demand; the variation of the above metrics is presented in the current work for each type of elevator, taking into consideration the usage intensity. In addition, the applicability and the effect of appropriate techniques were studied, in order to achieve reduction in the energy consumption and the upgrade of the energy class. The examination of the energy consumption goals, as well as the effectiveness of any proposed energy saving technique, was performed taking into consideration the technical characteristics and the usage of each elevator.
The performed analysis (the results of which are depicted in Figs. 6,7,8,9,10,11,12,13,14,and 15) indicated that a 20-40% of energy saving can be achieved by applying appropriate techniques for the restraint of the standby and travel energy consumption. It is worth mentioning that the upgrade of a significant number of the examined lifts to class A is a realistic goal. The above results depended on the cost and the flexibility of the necessary technical modifications, since changes to equipment and software that operate successfully and safely were not recommended. To this direction, the adoption of a stricter legislation for the energy consumption of the lifts and leverage policies for the installation of high energy class lifts can prove a valuable contribution. Additionally, the review of the VDI 4707 guideline was proposed, in order to include the necessary test procedures that warrant the limitation of the energy consumption by applying flexible decision making systems.
Concluding, the results of the current work can be a guide for other lift manufacturers, in order to achieve adequate energy saving, so a new generation of lifts can be developed, which will have positive consequences to the environment, electrical distribution networks, and economy.  Energy Efficiency (2017) 10:945-956